THE SIBERIAN MIGRATION. 227 



and one inter-glacial period is, I think, more gene- 

 rally adopted. Professor Geikie's four periods seem 

 to me to have originated i-n a desire to correlate 

 the British pleistocene deposits with the continental 

 ones, and at the same time to retain the old view of 

 the inter-glacial position of the Forest-Bed. The 

 two theories agree in so far as that in both the 

 glacial conditions culminate in a maximum glacia- 

 tion, followed by a more temperate phase of climate, 

 with consequent retreat of the ice-sheets, and finally 

 by a renewed advance of the glaciers. 



We are told that there is not the slightest doubt 

 about it that a marked but gradual decrease of 

 temperature took place all over Europe either 

 during the beginning of the Pleistocene or towards 

 the end of the Pliocene Epoch. 



We might reasonably suppose, then, that a similar 

 climatic effect was produced in Siberia, in con- 

 sequence of which the fauna would have been 

 obliged to retreat from the extreme northern lati- 

 tudes southward. No doubt great efforts would 

 have been made by the members of the Siberian 

 fauna at any rate by those possessing strong 

 power of locomotion to extend their range in 

 other directions. But we have no evidence that 

 a migration from Siberia came to Eastern Europe 

 at that time. It seems, therefore, as if the barrier 

 referred to by Brandt, Koppen, Boyd Dawkins, 

 and others (p. 222), had existed at this time. This 

 would have effectually prevented an overflow of 



